The technical field of the invention is that of installations using infrared radiations for detecting, in the atmosphere or under water, micro-leaks of gas escaping for example through welding seams joining up plates to form a wall.
The method which consists in using the detection of the near-infrared ranging between 2 and 5 microns, with a camera for displaying temperature differences, is nowadays wellknown.
The application of this method to the detection of leaks consists in displaying thermal gradients caused by the expansion of a gas after its passage though an orifice of very small cross-section. Such application may take place in the atmosphere or under water.
When performing the detection under water, two fundamental problems have to be solved:
(a) first, there must not be any aqueous shield between the screen and the plate to be examined so that the infrared radiation is not absorbed; and
(b) second, as heat diffusion inside the metal is very high, it is necessary to overcome it by a technical process. One accepted solution is to collect the fluid flow with a device permitting an adiabatic expansion.
In the case of small perturbations, it is possible, with the theory of internal variables which takes into account the two principles of thermodynamics and the FOURIER law, to write: EQU C.T.=divergent (k grad T)-T.alpha..sigma.+D+.phi.
C: is the volumetric specific heat PA1 T: is the absolute ambient temperature PA1 K: is the heat conductivity PA1 .alpha.: is the thermal expansion PA1 94 : is the stress PA1 D: is the intrinsic dissipation PA1 100 : is the density of the sources of heat.
In the equation developed hereinabove, the term "divergent (K grad T)" is very important in the case of metal, because of the value of coefficient K.
In order to solve the arising problem, the difficulty was divided with a device permitting to increase the value of the terms of the sources of temperature variations.